(1) Field of the Invention
The present invention relates to a semiconductor light emitting device, and more particularly to a semiconductor light emitting device which emits blue-green or blue light and in which ohmic characteristics are improved.
(2) Description of the Related Art
In recent years, extensive research and development work is being made on a blue-green (or blue) light emitting device, especially on a semiconductor blue-green laser. This is due to the improvement in p-type doping. As noted in the report concerning the operation under pulsed current injection at 77 K by M. A. Haase et al. of 3M in Applied Physics Letters, 1991, Vol. 59, No. 11, pp 1272-1274, the p-type doping had long been one of the most significant obstacles. This is followed by reports on remarkable achievement, such as a report on similar blue-green laser characteristics by researchers in Brown University and Purdue University published in Applied Physics Letters, 1992, Vol. 60, No. 17, pp 2045-2047 and a report on tile continuous wave operation of a blue laser diode at 77 K by researchers in Sony Corporation published in Electronics Letters, 1992, Vol. 28. No. 19, pp 1798-1799.
However, a great problem in the blue-light-emitting device today is that, because the electronic affinity is large in II-VI materials, no satisfactory ohmic electrode has been found and heat generation has been an obstacle for the improvement of the required characteristics. As an example of a blue-green laser device, FIG. 1 shows a structure which was disclosed in the report by Haase et al referred to above. In this structure, a gold electrode is directly bonded to a II-VI material so that the value of the turn-on voltage of the diode is very large. Also, in Japanese Patent Application Kokal Publication No. Hel 1-87885, Kamata et al propose an arrangement wherein the II-VI material and the electrode sandwich between them a GaAs layer, or a single crystal and a compound of III-V materials such as a GaP, InP. FIG. 2 shows in sectional view a semiconductor light emitting diode proposed by Kamata et al. With this structure, too, in the case of the former, that is, the GaAs layer, there is a problem in that the current injection is hindered by spikes due to heretojunction between the GaAs and the II-VI material. In the case of the latter, although some improvement is seen over the case wherein the electrode is bonded directly on the II-VI material, there are problems in that high ohmic resistance still remains and that the surface treatment involves difficulties because the GaInP is easily eroded by, for example, hydrochloric acid.